A. Measurement of Departures from an Inertial Frame
Instruments which can sense departures of their own reference frame from an "inertial reference frame" are of important practical and commercial use, for example, in the area of inertial navigation and guidance. Such departures are limited by Chasle's and Euler's Fixed-Point Theorems of classical mechanics to two basic kinds: rotations, and accelerations. The inertial effects of local accelerations (via Einstein's Equivalence Principle, which experimentally is known to hold to very high accuracy) are indistinguishable from those due to gravitational fields. The effects of simultaneous rotation and acceleration are interlinked via the Coriolis and centrifugal forces.